hypothesis published in January 2009 raised a lot of media interest;
Revival of anthropogenic models of megafauna extinctions consisted of further confirmations of the timing of megafauna extinctions and new mathematical models, which made overkill appear to be a reasonable mechanism for the extinctions. John Alroy argues that the coincidence of evidence for large human populations in the Americas by at least 13,400 years before present and confirmation of human hunting of at least some megafauna makes the overkill hypothesis plausible (3). Alroy therefore constructed a mathematical model of 41 prey species, some of which went extinct and others of which survived the end of the Pleistocene. He incorporated differences in prey body mass, geographic ranges, population densities, population growth rates (as predicted by body mass), rates of primary production and caloric values of plants and small game food resources, human nutritional needs, maximal rates of increase for human populations, and the first appearance of significant human populations in the United States (3). Additional, less constrained components of the model included the number of humans entering the region, hunting ability, and hunting effort (it was assumed to be related to handling time and nutritional requirements and so “per capita kill rates never exceeded a low ceiling”). The single best fit scenario in the original paper correctly predicts the survival or extinction of 32 out of 41 species (78 percent). Extinction times are also accurately predicted. It also takes 410 years for human populations to exceed 10,000, which may mean that early human occupation was at levels unlikely to appear in the archaeological record. Alroy further states that if the model is modified so that humans were already present at low densities before the extinctions, rather than arriving as a small founding population, extinction rates increase (3).Grayson criticizes the model for assuming that no megafauna went extinct before Clovis times and states that only 15 of 35 extinct genera can be confirmed to have survived into Clovis times and that only mammoth kill sites are confirmed (9). Alroy counters that radiocarbon dates are scarce for all but six genera, all of which overlapped with Clovis, and (without directly referencing it) cites the Signor-Lipps effect which shows that extinctions which take place in rapid periods of time would appear to be staggered in the fossil record. Alroy also notes that kill sites are likely for mastodons and giant tortoises as well and that “kill sites for smaller species are not expected because smaller bones are fragile.” In addition, the model is conservative in predicting how the deaths of some animals came as the result of human hunting. For example, it predicts that only nine percent of M. columbi deaths were due to hunting, while in reality nine out of 61 fossil sites are associated with kills (15 percent) (3). Slaughter and Skulan also point out an unrealistically low rm (species specific growth constant) used for some species, and argue that this would make megafauna extinctions too likely in the model (10). Alroy accepts their correction, noting that it was a programming error, and thanks them as the corrected model actually increases the predicted number of extinctions from 27 to 29 (3).
as required by the human-induced blitzkrieg extinction hypothesis.
Johnson (2002) sought to identify characteristics of species that made them more likely to go extinct when compared with close relatives. Low reproductive rates were associated with both likelihood of extinction, and relatively large body size, but this was not a uniform threshold (10 kg in lemurs compared with 350 kg in bovid) (11). Low reproductive rates could have predisposed some species to extinction, even under low hunting pressures. Johnson goes so far as to state that the number of deaths due to human hunting necessary to cause the extinction of some species may have been so low that “archaeological evidence of killing would be very sparse and in many cases could well be effectively veiled by its rarity” (11). The percentages of survivors from arboreal or closed habitats when compared with extinct species from those habitats (68 percent versus 22.8 percent, pet al. also acknowledge that since almost all the slow-breeding survivors in Australia, the Americas and Madagascar are nocturnal, arboreal, alpine, and/or deep forest dwellers, climate change alone seems less able to explain these extinctions than an overkill hypothesis (7).
Beck takes a paleontological approach by looking at the geographic patterns of megafauna extinctions in North America (2). Beck does support the timing of megafauna extinctions as coincident with Clovis technology (12-10 thousand years ago), but attempts to test the blitzkrieg models by looking at the distribution of the most recent remains of megafauna. If the blitzkrieg model is correct and humans advanced from an ice-free corridor in southwestern Canada, eliminating megafauna as they expanded into the United States, then megafauna would have been more likely to survive to later dates further from the initial point of intrusion. This implies that even though the fossil record is unlikely to preserve the last individual or even population of a particular species, statistically more terminal sites should be found in the Southern and Eastern portions of the ranges of megafauna. Only three of 11 megafauna taxa sampled by Beck have terminal sites in the southeastern halves of their ranges (2). Again this seems to refute the Clovis blitzkrieg model quite nicely, but rejecting this model of overkill does not mean that humans or even human hunting were not implicated in megafauna extinctions. Interestingly, Beck notes that the killing front he presumes is necessary for an overkill hypothesis is not needed by a mathematical model constructed by Whittington and Dyke (8), but that this model requires longer coexistence between humans and megafauna, which is precisely what Meltzer (1) uses in an attempt to refute the overkill hypothesis in general.
Quaternary extinction event - Wikipedia
Subsequent stratigraphic charcoal studies have established that fire was an integral part of Madagascars ecology long before human arrival and thus the Great Fire hypothesis of extinction was undermined in that case (Burney 1987).
09/01/2018 · What is the Overkill Hypothesis
The correlation between human immigration into the Americas, the invention of Clovis spear points and the extinctions of megafauna (large mammals exceeding 44 kg) at the end of the Pleistocene has caused many to implicate human hunting in the event. Correlation, however, does not necessarily imply causation. The close of the Pleistocene also marked a prolonged period of global warming and climate changes. In the 1990s the “overkill” hypothesis fell out of favor both because of its original ties to the “Clovis First” hypothesis (1) and paleontological evidence regarding the pattern of megafauna extinctions (2). At the turn of the millennium however, mathematical models (3,4), indirect evidence for human and megafauna occupations (5), and evidence for the reality of Clovis specialization as hunters of large game (6) made human hunting a viable hypothesis again. Recently Barnosky et al. argued that humans caused extinctions through multiple synergistic effects, and were likely responsible for extinctions in North America and Australia (although both the timing of human immigration and the extinctions in Australia have yet to be firmly determined), whereas extinctions in Europe were likely due primarily to climate change (7). Barnosky et al. also argue that megafauna extinctions in South America have yet to be properly studied, but are coincident with both human immigration and climate change (7). The debate however is far from closed.